1. Field of the Invention
The present invention relates to a method of forming a solder film. More particularly, this invention relates to a method suitable for forming on a printed circuit board a solder circuit which is a solder layer formed beforehand on a pad of a metallic circuit of the printed circuit board for easy mounting of electronic parts thereon, or for forming of a solder layer on an electronic part.
2. Description of the Related Art
In recent years, printed circuit boards (also referred to as printed boards) were developed which comprise an insulating substrate, such as a plastic substrate (including film), a ceramic substrate, or a metal substrate coated with a plastic or other substance, and an electronic circuit formed thereon; and a technique of soldering an electronic part such as an IC element, semiconductor chip, resistor, or capacitor to the circuit surface of such a printed board to constitute an electronic device is being widely employed.
In view of recent environmental problems, it is recommended to use Pb-free solder, i.e., a solder alloy comprising no lead. Correspondingly, Pb-free solder is being developed to be widely used. Among Pb-free solders, especially desirably, solder alloys such as Sn-Ag group and Sn-Zn group are being noted.
In producing the above-described circuit device having electronic parts mounted thereon, the ordinary method for soldering a lead terminal of the electronic part to a predetermined pad of the circuit comprises forming a thin solder layer beforehand on either or both of the pad and the lead terminal, positioning the electronic part, and then melting (reflowing) the thin solder layer(s) to solder the electronic part on the circuit.
For the formation of the solder circuit (thin solder layer), such methods as plating, dipping (immersion) in a solder bath, and printing of a solder powder paste have been employed. However, as the trend toward the increase in mounting density, solder circuits have been required to have even finer patterns and there also are desires for improvements in working efficiency and on-specification rate and for circuit pattern miniaturization. It is, therefore, becoming difficult to cope with these requirements with the above methods.
Among those conventional methods, the plating method is applicable for formation of solder circuits having highly precise and fine patterns.
The plating method is classified into electroplating and electroless plating. Use of electroplating, however, encounters difficulties in attaining electrical conductivity because, in actual printed circuit boards, the parts in which solder circuits are to be formed are present independently from the circuit parts in most cases. On the other hand, electroless plating has a technical problem that it is difficult to obtain a thick solder layer having a thickness necessary in practical use, although the problem concerning electrical conductivity in the electroplating is overcome.
Further, in electroless plating, the thickness of the solder pattern obtained is generally 5 xcexcm at the thickest. Also, it is difficult that each independent solder pattern cannot be obtained in a thickness for obtaining a sufficient conductivity. On the other hand, as a means for ensuring an amount of solder, an electroplating method can be used. However, the lower limit of a pattern pitch being able to be made is 400 xcexcm (pattern width L=200 xcexcm, space between adjacent two patterns S=200 xcexcm). A smaller pattern than this pattern size can easily cause a short circuit and thus cannot be put into practical use.
In U.S. Pat. No. 5,532,070, in order to solve the above-described problem, there is disclosed a method wherein a Sn layer and a Pb layer are separately formed according to an electroless plating method, then the layers separately formed are melted to form an alloy, whereby a solder pattern having a pattern pitch of 150 xcexcm and a solder thickness of 100 to 150 xcexcm is formed. However, a solder pattern which can be formed by this method is limited to Sn-Pb solder. This is because since the method described in U.S. Pat. No. 5,532,070 comprises plating a Sn layer and a Pb layer separately, thereafter melting the resulting layers to form an alloy, the method can be readily applied to a solder alloy comprising metals having a low melting point such as Sn and Pb as constituent elements, but is hardly applied to a solder alloy comprising Ag, Zn or the like, which is noted as a Pb-free solder. Namely, Sn has a melting point of 232xc2x0 C. and Pb has a melting point of 327xc2x0 C., while the melting points of Ag and Zn are 961xc2x0 C. and 419xc2x0 C., respectively, which are higher than that of Pb, and, therefore, heating in forming an alloy of these metals with Sn may cause breakage or deterioration of a board or an electronic part.
A method of electrostatically applying flux-coated solder powder particles on a circuit part has been proposed in JP-A-3-50853. (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d.) However, this method is still unable to easily produce a high-accuracy fine pattern.
Another method for the solder circuit formation has been proposed in JP-A-4-10694, which comprises applying a flux on a circuit part by printing, adhering a powdered solder to the flux-printed part, melting the solder by heating it to a temperature not lower than the melting point of the solder, and then blowing a gas on the solder melt to level it to thereby form a solder circuit. This method is disadvantageous in that a high degree of skill is required because high-precision printing of a flux on a pad is difficult and in addition there is a fear of bridging between patterns spaced at a minute gap during the leveling of the solder melt.
Moreover, in a printing method, pattern accuracy cannot be obtained. When the thickness of the pattern printed is 30 rim, a deviation of the thickness a is 5 xcexcm.
Further, since in the printing method more accurate mask alignment is required, the practical pitch of the pattern is 250 xcexcm at smallest (pattern width L=125 xcexcm, space between adjacent two patterns S=125 xcexcm).
Also in the case where a solder coat (solder layer) is formed on a lead terminal of an electronic part, there are the same technical problems as those in the formation of solder circuits.
As a result of extensive studies made in order to improve the precision of solder patterns which are required to be fine, it has been found that the plating method, which has attained the highest precision, still has several problems and has to be improved in working efficiency and other respects.
In Pb-free solder alloys such as Sn-Ag group, Sn-Zn group, and the like, an object of the present invention is to provide a method of efficiently forming a precise, fine, bridge-free solder film pattern on a metallic circuit or on an exposed metal of an electronic part by a simple procedure without the necessity of troublesome operations such as positioning.
It has been found that a certain chelate compound acts on an exposed metallic surface to render it tacky, which makes it possible to precisely adhere a powdered solder only to the resulting tacky surface. The present invention has been attained by the above discovery. That is, the present invention is a method for forming a solder film, which comprises selectively imparting tackiness to only an exposed metallic part of a printed circuit board or electronic part by means of a tacky layer-forming solution containing at least one compound (tackiness-imparting compound) selected from benzotriazole derivatives, naphthotriazole derivatives, imidazole derivatives, benzoimidazole derivatives, mercapto-benzothiazole derivatives, benzothiazole thiofatty acid derivatives, and triazine derivatives, adhering a powdered solder to the resulting tacky part, and then melting the solder by heating to form a solder film on the part.
In Pb-free solder alloys such as Sn-Ag group, Sn-Zn group, and the like, in particular, a practical pattern having a pitch being not more than 200 xcexcm (pattern width L=not more than 100 xcexcm, space between adjacent two patterns S=not more than 100 xcexcm) and a deviation of the thickness a being not more than 2.4 xcexcm can be obtained easily. Therefore, in Pb-free solder alloys such as Sn-Ag group, Sn-Zn group, and the like, according to the present invention, such a precise, fine and accurate solder pattern that conventional method cannot provide can be obtained easily.
The reasons why a precise, fine and accurate solder pattern can be obtained by the method of the present invention are as follows.
By selecting an appropriate particle size of a solder powder, a corresponding thickness of the solder pattern can be obtained easily and accurately. Also, the solder pattern can be prevented from causing a bridge (short circuit). By selecting a maximum particle size of the solder powder so as to be half of the space between the adjacent two patterns, an accurate, fine, precise and reliable solder pattern can be obtained.
The reasons why the thickness deviation of the solder pattern is small in the method of the present invention are as follows.
Since the particle size of a solder powder used in the method of the present invention is not less than 10 xcexcm and is sufficiently large, and the thickness of the tacky layer is not more than 1 xcexcm , solder powder can be adhered to the tacky layer in a one particle layer without laminating a plurality of particle layers of powder.